Host Range of Itersonilia perplexans and Management of Itersonilia Petal Blight of China Aster

Isolates of Itersonilia perplexans were tested for pathogenicity on 14 different plant species. Isolates from China aster and sunflower were each pathogenic to the other host and both isolates were pathogenic to chrysanthemum and gerbera, causing moderate to high incidences of flower or seedling blight in these hosts. China aster and sunflower isolates were not pathogenic to burdock, but an isolate from burdock was pathogenic to florists' chrysanthemum and gerbera. In a survey of weed species near a cut-flower production facility in southwest Florida, I. perplexans was found in only one weed species, Emilia fosbergii. However, two isolates from this weed host produced severe petal blight symptoms when inoculated onto China aster. Myclobutanil, potassium bicarbonate, and propiconazole were highly effective in reducing disease severity of petal blight in China aster, while azoxystrobin reduced disease moderately in fungicide evaluation trials. Accepted for publication 28 July 2006. Published 18 October 2006.

A Blight Disease of Dill in California Caused by Itersonilia perplexans

In 2000, a foliar disease was observed in commercial field plantings of dill (Anethum graveolens) in coastal California. Initial symptoms consisted of a gray-green discoloration and wilting of the tips of dill leaves. As disease developed, many of the leaves discolored and collapsed, which gave the foliage a blighted appearance and made the leaves unsuitable for harvest. A fungus was consistently isolated from symptomatic leaves. Slow growing colonies were cream-colored, velutinous, and flat with minimal aerial mycelium. Mycelium was hyaline and had clamp connections. Ovoid to subglobose sporogenous cells were abundant and measured 11 to 14 × 12 to 16 μm. Spores were ballistospores that were bilaterally symmetrical, lunate, 16 to 18 × 12 to 13 μm, and germinated with hyphae or secondary ballistospores. The fungus was identified as Itersonilia perplexans Derx (1,2). Pathogenicity was tested by preparing spore suspensions (1 × 104) and spray-inoculating potted dill plants. Plants were placed in a dew chamber for 24 h and then maintained in a greenhouse (23 to 25°C). After 10 days, leaves showed symptoms similar to those observed in the field and the same fungus was reisolated. Control plants sprayed with water did not develop symptoms. Inoculations were repeated and the results were the same. In another experiment, sets of dill plants and flowers of five cultivars of potted florist's chrysanthemum (Dendranthema × grandiflorum [= Chrysanthemum × morifolium]) and one cultivar of Leucanthemum paludosum (= C. paludosum) were spray-inoculated with several dill isolates and incubated as described previously. Dill plants developed symptoms and the fungus was reisolated. However, flowers of the six chrysanthemum cultivars did not develop any petal blight symptoms. Inoculations were repeated and the results were the same. These findings are consistent with previous studies that indicate I. perplexans consists of pathogenic forms that are host-specific to either chrysanthemum and closely related Compositae or to Apiaceae plants such as dill, carrot (Daucus carota subsp. sativus), parsley (Petroselinum crispum), and parsnip (Pastinaca sativa) (1,2,3). To our knowledge, this is the first report of I. perplexans on dill in California, and also appears to be the first record of this disease in North America. References: (1) T. Boekhout. Mycol. Res. 95:135, 1991. (2) T. Boekhout et al. Can. J. Microbiol. 37:188, 1991. (3) A. G. Channon. Ann. Appl. Biol. 51:1, 1963.

Petal Blight of Sunflower Caused by Itersonilia perplexans

In February 1999, a cut-flower grower from Florida reported post-harvest losses of approximately 25% in Helianthus annuus (sunflower) cvs. Sunrich Orange and Sunbright. Symptoms began as small, pinpoint brown lesions on the petals, which enlarged and coalesced, leading to necrosis of the entire blossom. Symptomatic petals were surface disinfested for 30 sec with 0.5% NaOCl and suspended over plates containing acidified 25% potato-dextrose agar. Petals adhered to lids of the petri dishes by the surface tension of the water on the petals. Itersonilia perplexans Derx was consistently isolated. Inoculation of sunflower with I. perplexans was achieved using three detached, mature blossoms of the cv. Sundance Kid with their pedicels placed in sterile water. Each blossom was inoculated with 3.5 ml of a spore suspension (1 × 105 spores/ml) applied as a spray. Three noninoculated control blossoms were sprayed with sterile, distilled water. Blossoms were incubated in a controlled environment chamber at near 100% RH, a constant temperature of 18°C, and a 12-h photoperiod. Three days following inoculation all of the inoculated blossoms were symptomatic, with necrosis ranging from 50 to 100% on individual petals. I. perplexans was recovered from all of the inoculated blossoms. Noninoculated controls remained symptomless, and I. perplexans was not recovered. I. perplexans has been reported to cause lesions on the cotyledons, first true leaves, and the hypocotyl region of sunflower seedlings in Canada and Uruguay (1,2). However, this is the first report of post-harvest petal blight in sunflower caused by I. perplexans and the first report in the United States of infection of sunflower by I. perplexans. References: (1) W. E. Sackston. Can. Phytopath. Soc. 1952:22, 1953. (2) W. E. Sackston. Phytopathology. 48:108, 1958.

Petal Blight of Callistephus chinensis Caused by Itersonilia perplexans

From November 1997 to March 1998, a cut-flower grower in southwest Florida reported post-harvest losses due to a petal blight of 10 and 50%, respectively, in China aster (Callistephus chinensis (L.) Nees) cultivars Rainbow Mix and Matsumoto Lavender. Initial symptoms observed included pinpoint, bleached lesions on petals. Enlargement of lesions resulted in bleached streaks on ray flowers and eventually necrosis of entire blossoms. Symptomatic petals were surface disinfested in 0.5% NaOCl and suspended on the lids of plastic culture plates over acidified potato dextrose agar (APDA). Itersonilia perplexans Derx was isolated consistently from diseased petals but not from symptomless controls. Confirmation of the pathogenicity of I. perplexans used three detached, mature flowers of China aster cv. Matsumoto Rose and seven mature flowers of Bidens bipinnata L., a common composite found near the production site. The pedicels of flowers were placed in sterile, deionized water, and 1 ml of a spore suspension (1.2 × 104 spores per ml) of I. per-plexans was sprayed on aster flowers, and 0.5 ml was applied to flowers of B. bipinnata. An equal number of noninoculated control flowers were sprayed with sterile, deionized water only. The flowers were incubated in a controlled environment chamber at near 100% relative humidity, a constant temperature of 18°C, and a 12-h photoperiod. Bleached spots were detected on the petals of Matsumoto Rose flowers 10 days after inoculation, and the fungus was reisolated on APDA. Control asters remained symptomless, and were negative for I. perplexans. Brown blotches appeared on the petals of B. bipinnata flowers 12 days after inoculation, and infection by I. perplexans was confirmed by reisolation. The fungus was not detected in noninoculated B. bipinnata flowers, including some that exhibited brown discoloration. A survey was conducted in early March 1998 to examine the incidence of Itersonilia petal blight in China aster and the occurrence of the fungus in B. bipinnata. Fifty flowers of each of four China aster cultivars were collected at random without regard to symptoms. Approximately 20 flowers of B. bipinnata exhibiting brown spots were also collected near this site. The fungus was isolated at incidences of 18, 6, 8, and 6% in flowers from aster cultivars Rainbow Mix, Matsumoto Blue, Matsumoto Pink, and Matsumoto Scarlet, respectively. I. perplexans was not detected in B. bipinnata flowers collected from the field during this period. I. perplexans previously was detected in florists' chrysanthemum (Chrysanthemum × morifolium Ramat.) at the same cut-flower production site in January 1973 during a period of uncharacteristically cool weather (2). The weather during the recent outbreak of Itersonilia petal blight in aster was also unusually cool and rainy and resulted from the “El Niño” weather pattern prevalent during the period. I. perplexans has been found to over-season on many weeds in Great Britain, particularly on those in the Compositae (1). Experimental transmission of I. perplexans to B. bipinnata suggests that it and other wild composites may serve as reservoirs of the fungus in Florida. References: (1) D. G. Gandy. Trans. Br. Mycol. Soc. 49:499, 1966. (2) J. J. McRitchie et al. Plant Dis. Rep. 57:181, 1973.